[PSander]

Hey,
I try to calculate heat transfer between a fluid in a pipe (water) and the encircling pipe and soil, using OF6 and the chtMultiRegionFoam solver for a steady state calculation.
The model crashes with the fatal error of exceede Maximum number of iterations as well the Courant-number drops below 0, what I consider as part of the problem.

Region: fluid Courant Number mean: -0.0910146934732 max: -0.00351789034512
Region: pipewall Diffusion Number mean: 2.67864717358e-06 max: 0.000233164935363
Region: soil_1 Diffusion Number mean: 5.82937716003e-06 max: 7.56997601431e-05
deltaT = 4.68264248705
Time = 10


Solving for fluid region fluid
DILUPBiCGStab:  Solving for Ux, Initial residual = 0.929863015554, Final residual = 1.9550289574, No Iterations 1000
DILUPBiCGStab:  Solving for Uy, Initial residual = 0.978339899353, Final residual = 1.05520532405, No Iterations 1000
DILUPBiCGStab:  Solving for Uz, Initial residual = 0.999766111841, Final residual = 8.79009066096e-05, No Iterations 1000
DILUPBiCGStab:  Solving for e, Initial residual = 1, Final residual = 1386552.30948, No Iterations 1000


--> FOAM FATAL ERROR: 
Maximum number of iterations exceeded: 100

I read in the forum to check the mesh but I could not find any error.


If I try the same model with Euler instead of steady state, it crashes with a negative initial temperature and a extrem high courant-number (I fixed maxCo to 1 in controlDict, but it seems to be ignored).

Region: fluid Courant Number mean: 28.0700323842 max: 3023.8583768
Region: pipewall Diffusion Number mean: 9.88492992534e-05 max: 0.00860441445906
Region: soil_1 Diffusion Number mean: 2.15119726494e-05 max: 0.000279352514867
deltaT = 5.92803286081e-06
Time = 388.886


Solving for fluid region fluid
diagonal:  Solving for rho, Initial residual = 0, Final residual = 0, No Iterations 0
DILUPBiCGStab:  Solving for Ux, Initial residual = 0.0861844749054, Final residual = 3.10312044775e-07, No Iterations 2
DILUPBiCGStab:  Solving for Uy, Initial residual = 0.215185941109, Final residual = 1.92800159784e-07, No Iterations 2
DILUPBiCGStab:  Solving for Uz, Initial residual = 0.0288023464831, Final residual = 1.83257387023e-08, No Iterations 2
DILUPBiCGStab:  Solving for e, Initial residual = 0.256558566591, Final residual = 8.40025755292e-07, No Iterations 2


--> FOAM FATAL ERROR: 
Negative initial temperature T0: -845.998590083

In another attempt I already reduced my mesh to an easier version with only one solid part, but I get still the same probelm with negative initial temperatures or exceeded number of iterations.

Given that my model had none of those problems in OF7, I believe that the boundary conditions should be fine, so I dont know how to fix this issue.
Do you have some hint how to solve it?